1. Field of the Invention
The present invention relates to a control apparatus and a method for a forklift provided with a hydraulic pump and a hydraulic motor for operating a lift cylinder for lifting up and down a fork.
2. Description of the Related Art
Conventionally, as a cargo apparatus for a forklift used for loading and unloading a baggage, ones being operated by hydraulic pressure have been employed. The hydraulic circuit of such a cargo apparatus is configured as shown in FIG. 6, for example. That is, a lift cylinder 3 for lifting up and down a fork 1 is coupled to a hydraulic pump 9 through a fuse valve 5 and a lift control valve (hereinafter called as a lift valve) 7 which is changed over between a lift-up side (the left side in FIG. 6) and a lift-down side (the right side in FIG. 6) by the operation of a lift lever. The hydraulic pump 9 is driven by a hydraulic motor 11, whereby oil reserved within an oil tank 13 is supplied to the lift cylinder 3. A reference numeral 8 depicts a check valve provided at a path between the hydraulic pump 9 and the lift valve 7.
Further, as shown in FIG. 6, a reach cylinder 15 for moving the fork 1 in the forward and backward direction is coupled to a reach control valve (hereinafter called as a reach valve) 17 which is changed over between a reach-in side (the left side in FIG. 6) and a reach-out side (the right side in FIG. 6) by the operation of a reach lever. The reach valve 17 is coupled to the hydraulic pump 9 through a check valve 19 and also coupled to the oil tank 13.
Further, as shown in FIG. 6, a tilt cylinder 21 for tilting the fork 1 is coupled to a tilt control valve (hereinafter called as a tilt valve) 23 which is changed over between a tilt-up side (the left side in FIG. 6) and a tilt-down side (the right side in FIG. 6) by the operation of a tilt lever. The tilt valve 23 is coupled to the hydraulic pump 9 through a check valve 25 and also coupled to the oil tank 13.
Furthermore, as shown in FIG. 6, a relief valve 27 is disposed between a coupling tube on the oil tank 13 side of the reach valve 17 and a coupling tube on the oil tank 13 side of the tilt valve 23. The fuse valve 5 and a flow rate control valve 29 are disposed between the coupling tube of the lift cylinder 3 and the coupling tube of the lift valve 7. In particular, when the lift cylinder 3 is lifted down, a flow rate of the oil to be fed back to the oil tank 13 is controlled by the flow rate control valve 29. A check valve 31 is coupled in parallel to the flow rate control valve 29.
However, according to the aforesaid conventional configuration, when the fork 1 is lifted down from a high position, the oil is merely fed back from the lift cylinder 3 to the oil tank 13 through the fuse valve 5 and the flow rate control valve 29 without using the potential energy of the fork 1. Thus, this potential energy is wasted.
In particular, when a baggage is loaded on the fork 1, since the weight of the baggage is added to the dead weight of the fork 1, the potential energy becomes very large. Thus, it is desired to effectively utilize such a large potential energy.
Further, the speed or the maximum speed of the fork at the time of the lift-down thereof is determined by the dead weight of the fork 1, the weight of a baggage, an opening degree of the lift valve 7 and the characteristics of the flow rate control valve 29. Thus, particularly, in the fully opened state of the lift valve 7, there arises a problem that it is difficult to control the speed at the lift-down operation to a desired speed.
However, conventionally, it has been considered to use a DC motor for the hydraulic motor and to regenerate the potential energy of the fork to a battery serving as a driving source, during the time of the lift-down operation. However, in the case of using the DC motor, since a regeneration control circuit is required in addition to a normal driving control circuit, the configuration of the control circuit becomes complicated and expensive. Thus, there arises an inconvenience that although the potential energy can be utilized, economical efficiency is degraded.